The present invention relates to brazed-plate heat exchangers, of the type comprising at least one fluid passage extending in a first direction, provided with exchange fins substantially parallel to the first direction and comprising, in an intermediate area of its length, at least one redistribution zone associated with at least one lateral collector and provided with deflection fins extending along at least a second direction forming an angle with the first direction, this redistribution zone occupying the entire width of the passage, crosswise to the first direction, the deflection fins all communicating with exchange fins.
The document xe2x80x9cThe standards of the brazed aluminum plate-fin heat exchanger manufacturer""s associationxe2x80x9d, Alpema, first edition 1994, describes various heat-exchanger configurations of this type.
More particularly, although not exclusively, the present invention relates to heat exchangers for vaporizing pressurized liquids, in particular pressurized cryogenic liquids.
Air separation apparatus of the xe2x80x9cpumpxe2x80x9d type produces at least one gas from air (generally oxygen) in liquid form which is pressurized by a pump before its vaporization, thus enabling the compression of the pure products in gaseous form by a compressor to be avoided, which is particularly beneficial, both economically and technically, and safer with oxygen. The pressurized liquid is warmed and vaporized in a heat-exchange line which generally consists of one or more exchanger bodies, typically of the plate type and made of brazed aluminum. At the point in the exchanger where the pressurized liquid stops vaporizing, there are abrupt variations in the exchange coefficient of this fluid resulting from flow fluctuations causing the product to pass from a two-phase mixture state, with a high exchange coefficient, to a purely gaseous state, with a much lower exchange coefficient. This leads to an appreciable local variation in the temperature of the wall or plate and of the exchanger fins. When the temperature difference between the fluid being vaporized and the adjacent heat-transfer fluids is appreciable, possibly greatly exceeding 10xc2x0 C., and typically between 10xc2x0 C. and 30xc2x0 C., the local temperature of the wall and of the fins may vary rapidly and abruptly, typically by 5 to 15xc2x0 C. in a repetitive manner, which is enough to cause thermal fatigue phenomena.
When these fluctuations occur in a homogeneous zone of the exchanger, the fatigue remains very limited. On the other hand, the fatigue becomes considerable when the fluid stops vaporizing opposite deflection fins contained in the heat-transfer fluid passages. This situation corresponds, for example, to an outlet of partially cooled air intended for expansion in a turbine in order to keep the plant cold. This is because the thermal fatigue phenomena are aggravated by the thermal and mechanical heterogeneities of the structure, the distribution or deflection fins being spaced further apart and thicker, and therefore stiffer, than the exchange fins, in order to provide a lower resistance to the passage of fluids while providing the desired mechanical strength. Generally, the deflection fins have a pitch which is greater than the exchange fins in order to reduce the pressure drop of the distribution zones, and consequently these deflection fins are thicker in order to withstand the pressure. Other unfavorable factors are the presence of dead spots, and possibly of separation bars, in the redistribution zones.
Thus a tendency for the plates to fail in the boundary regions of the deflection fins is observed.
These thermal fatigue phenomena are particularly difficult to prevent in the intermediate redistribution zones where heat-transfer fluid is removed from the container and/or is introduced into it. This is because, depending on the type of market for the plant, the end-of-vaporization point varies considerably in the adjacent passages, and it is not possible to counteract a large height in the heating passages in order to position the redistribution zones.
The object of the present invention is to provide a heat-exchanger structure comprising at least one redistribution zone avoiding, in particular, the drawbacks mentioned above.
To achieve this, the subject of the invention is an exchanger of the aforementioned type, characterized in that the deflection fins have substantially the same stiffness as the exchange fins.
With the arrangement according to the invention, it is thus possible to eliminate the dead spots in the redistribution zone, which prevents having the surface of the exchanger follow only the temperature of the vaporized fluid, and provides good thermal and mechanical homogeneity between the redistribution zone and the exchange zones. The thermal fatigue phenomena are thus considerably reduced.
With such an arrangement, the distribution zone has a stiffness comparable to that of the rest of the passage, and heat exchange with the fluids whose temperature changes the least and the most slowly is enhanced.
According to one characteristic of the invention, the deflection fins have substantially the same geometric proportions as the exchange fins, that is to say they have relatively little stiffness and provide a good heat-exchange surface. In this context, the use of an internal separation or sealing bar usually placed between the upstream and downstream part of a redistribution zone is preferably precluded.
In contrast, according to one aspect of the invention, the deflection fins and the exchange fins may be of different types (perforated and nonperforated, and/or serrated and nonserrated).
The object of the present invention is also to propose the application of such an exchanger to the vaporization of pressurized liquids, typically to the vaporization of pressurized cryogenic liquids.
For this reason, another subject of the invention is an air distillation apparatus producing at least one pressurized cryogenic liquid and equipped with at least one such exchanger serving to vaporize this liquid.